Originally Posted by Angel

My father always told me to find an old walkman or handheld AM radio - tune it as far down as it will go, hold it close to the switchplate and listen to the volume of the AC hum that you hear - it'll get louder near any fixtures where there might be a loose connection.
The idea being that 'more 60hz will leak out' if there is a loose wire or flakey connection.

I thought that the theory on this was kinda flakey (radios have RF bandpass filters...) but it worked on my house - I found a screw on a light switch that was making good contact but was loose.

More to your point- screws and wire nuts will work loose first - if there are multiple light fixtures in line, there is probably a wire nut somewhere that connects everything. I'd start at the flakey fixture - I've seen more variance in light fixtures/connections than light switches.

-John